A conventional impact extrusion molding method will be described with reference to FIGS. 15 and 16(a) to 16(c). It should be noted that the same constituent elements are denoted by the same reference numerals throughout the drawings. FIG. 15 is a partially sectional explanatory view of a conventional impact extrusion molding apparatus. FIG. 16(a) is a front view of a slug, which is a material; FIG. 16(b) is a cross-sectional view of a semifinished product (intermediate molded article); and FIG. 16(c) is a perspective view of a molded article (product).
In FIG. 15, a dice 1 is attached to a dice holder 10, and a punch 2 is attached to a punch holder 20. The dice 1 and the punch 2 are provided at a slide and a bolster of a press (not shown), respectively.
In the conventional impact extrusion molding method, a slug 30, made of such as aluminum alloy or the like, as shown in FIG. 16(a), is set in the dice 1, and the punch 2 is caused to slide, forcibly, from above the slug 30, into the dice 1, by a predetermined distance, whereby a bottomed cylindrical product 3, as shown in FIGS. 15 and 16(c), is molded through a semifinished product (intermediate molded article) 31, as shown in FIG. 16(b).
The product 3 thus molded is discharged from the dice 1 by a rod 4, for example, which goes through the bottom of the dice 1, and the dice holder 10. When the product 3 is raised together with the punch 2 while being attached to the punch 2, the product 3 is stripped away by a stripper plate or the like, not shown. The rod 4 is fixed so as to contact the slug 30 during molding operation. After finishing the molding operation, the rod 4 is forced into the dice 1, making it possible to discharge the product 3 from the dice 1.
In the molding process stated above, when the punch 2 is forced into the dice 1 by a certain depth or more, metal flows from the center to the side directions, as indicated by arrows r shown in FIG. 16(b), on the bottom of the semifinished product 31, which is being molded as shown in FIG. 16(b), and the metal thus flowing to the bottom side surfaces of the semifinished product is caused to flow, as indicated by arrows r′, upward along the side surfaces of the semifinished product 31, toward the extrusion direction (opposite to the punch pressing direction) (which will be referred to as “metal-flow” in this specification).
The slug 30 may be a slug produced by cutting an extrusion molded rod material into slices, or a slug produced by blanking or cutting out a rolled plate material. Therefore, the metallographic structure of the outer surface portion of the slug 30, shown in FIG. 16(a), often differs in conditions from that of the bottom portion of the slug 30. If so, on the lower outer surfaces of the side of the molded product 3, metallographic structures having different conditions exist in a mixed state. As a result, as shown in FIG. 16(c), because of different working histories, a side surface region 3a, which differs in conditions, including surface glossiness and metallographic structure, from the other regions of the side surfaces, is formed.
Since such a region 3a, having different conditions, is concealed after the product 3 is subjected to a surface treatment, to coat the surface of the product 3 by coating or the like, it does not hamper the appearance of the product 3. However, when the surface treatment for coating the surface of the product 3 is not carried out, or when an anodizing, in particular, is conducted on the product 3, the region 3a having different conditions remains, with the result that the appearance of the product 3 is disadvantageously hampered.